Nowadays, more and more Liquid Crystal Display (LCD) color televisions (TVs) use Light Emitting Diode (LED) backlighting technology. Advantages of LCD color TVs using LED backlighting technology are long service life, energy saving, and convenient driving. There are two typical LED backlighting schemes. One is of a direct type, where a number of lights are placed directly under the screen. The other is of an edge type, where a number of lights are distributed around the screen and the light is evenly guided to the LCD screen through a light guide plate. Both of the above LED backlighting schemes are widely used, but the direct type has more advantages. Its primary advantage is the ability to enable the Local Dimming function. The so-called Local Dimming is to divide the lights behind the screen into multiple groups, such as 32 groups, 64 groups, or 128 groups. Each group is composed of several lights in series. Generally, the greater the number of groups and the finer the grouping, the better the dimming effect. The brightness of each group of lights is determined by the brightness of the picture, thereby enabling better display effects and making it more energy-saving.
Typical Local Dimming generally adjusts the brightness by way of adjusting both a Pulse Width Modulation (PWM) and a reference current of the LEDs. This however requires that each channel of a driving Integrated Circuit (IC) must be able to adjust the reference current and the duty cycle at the same time. In this case, a digital-to-analog converter is required for each channel of the driving IC, resulting in a high production cost of the driving IC. Thus, the current method of controlling the brightness is using a fixed maximum current and then adjusting the PWM according to this current. When using this method, however, the corresponding voltage will also increase when the LED current increases. Accordingly, the lighting efficacy of the LED lights will be reduced, which is not conducive to energy saving and increasing the contrast of the displayed picture.